CHAPTER 10
VHDL

The U.S. Department of Defense (DoD) supported
the development of VHDL (VHSIC hardware description language) as part of
the VHSIC (very high-speed IC) program in the early 1980s. The companies
in the VHSIC program found they needed something more than schematic entry
to describe large ASICs, and proposed the creation of a hardware description
language. VHDL was then handed over to the Institute of Electrical and Electronics
Engineers (IEEE) in order to develop and approve the IEEE Standard 1076-1987.
1 As part of its standardization
process the DoD has specified the use of VHDL as the documentation, simulation,
and verification medium for ASICs (MIL-STD-454). Partly for this reason
VHDL has gained rapid acceptance, initially for description and documentation,
and then for design entry, simulation, and synthesis as well.

The first revision of the 1076
standard was approved in 1993. References to the VHDL Language Reference
Manual (LRM) in this chapter--[VHDL 87LRM2.1, 93LRM2.2]
for example--point to the 1987 and 1993 versions of the LRM [IEEE, 1076-1987
and 1076-1993]. The prefixes 87 and 93 are omitted if the references are
the same in both editions. Technically 1076-1987 (known as VHDL-87) is now
obsolete and replaced by 1076-1993 (known as VHDL-93). Except for code that
is marked 'VHDL-93 only' the examples in this chapter
can be analyzed (the VHDL word for "compiled") and simulated using
both VHDL-87 and VHDL-93 systems.